By examining the cognitive effects of the surgical procedure necessary for implanting electrodes into the brain and the cognitive effects of chronic deep brain stimulation in patients with idiopathic Parkinson's disease (PD), invaluable knowledge will gained about the potential cognitive gains and losses that may result from this relatively new and unreaserched method for treating the motor symptoms of PD. Patients will be administered, following parkinsonian medication withdrawal, three alternate equivalent versions of a neuropsychological test battery for each of the three study conditions. By comparing baseline presurgical neuropsychological test performance to postsurgical stimulator off performance, the potential cognitive effects of the surgical procedure can be assessed. This procedure may not be appropriate for some patients if it is found that the surgical procedure results in a significant decrease in cognitive functioning. By comparing neuropsychological data from the postsurgical stimulator on and off conditions, the effect of chronic deep brain stimulation can be examined. The value of this procedure as a treatment for PD may be increased if it is found that by altering, to some extent, the neuropathology of PD, there is an improvement in cognition, as well as in motor functioning. Conversely, if chronic stimulation results in increased cognitive deficit, then not only will important information about this new procedure by gained, but a greater understanding of the differential role some subcortical structures play in cognition and movement will be obtained. Therefore, the information acquired in this study will add to the clinical utility of the deep brain stimulation procedure and to the understanding of brain/behavior neural mechanisms underlying the specific cognitive functions that are affected by PD.